This invention relates to a fixing device for fixing toner images in electrophotography and more particularly to such a fixing device characterized by the position on its heating roller where a temperature detector is disposed.
The fixing device for an electrophotographic copying machine and the like typically comprises a heating roller which is made of metal and is heated to a temperature of about 200.degree. C. and a compression roller which is disposed opposite to the heating roller and, when in operation, is compressed against it and follows its motion. The fixing process is effected with these rollers compressed against each other and rotating together, and a sheet of copy paper with a toner image transferred on its surface passes between these rollers. As the paper passes between the rollers, the toner is melted by heat and the image becomes fixed by the pressure of the rollers.
The heating roller is a hollow cylinder and holds a heater lamp at its center section. In addition, a temperature detector for the heater control such as a thermister is disposed in contact with the surface of the heating roller to detect its temperature. In order to maintain the heating roller at an appropriate temperature level for the fixing process, the power delivered to the heater lamp is controlled according to the output from this temperature detector.
In FIGS. 3A and 3B, a previously designed fixing device is illustrated with a heating roller 10, a compression roller 20 compressed against it, and a heater lamp 3 at the center of the heating roller 10. FIG. 3C shows the temperature distribution on the surface of the heating roller 10 by the operation of the heater lamp 3. It shows that the surface temperature is slightly higher near the ends and flat near the center, and it is because the rollers are intentionally so designed in order to compensate for the temperature drop at these places caused by the heat loss through the bearings for the heating roller 10. Since the thermal resistance of the roller housing is relatively high, however, the temperature distribution on the surface of the heating roller 10 does not always become flat, depending on the number of sheets of paper which pass through per unit time. FIG. 3C represents such a situation where temperature is relatively higher at the end sections. In order to be able to detect an average fixing temperature even if there is some such distortion in the temperature distribution, the temperature detector is disposed on the heating roller at a point which may be expected to be nearly in the middle of the temperature distribution. Because of the contact with the temperature detector, however, the surface of the heating roller sometimes becomes scratched and this may adversely affect the fixed image. In order to eliminate such a problem, it may be considered preferable to put the temperature detector at an end section of the heating roller but a representative value of the surface temperature cannot be detected at such a position, and it becomes difficult to accurately control the temperature of the heating roller.